In many magnetic flow meter applications, there is a substantial amount of 1/f noise that interferes with measurement of flow voltage at flow meter electrodes. This noise is higher at lower frequencies and decreases as the frequency increases. One way to improve the signal-to-noise Ratio (SNR) of the electrode voltage measurement is to increase the frequency of the coil drive current. By increasing coil drive current frequency from 5 Hz to 37 Hz, for example, the 1/F noise level decreases at the measurement frequency while the electrode voltage level stays the same. The SNR increases and more accurate flow measurements can be made. The noise floor at 5 Hz can be many times greater than the noise floor at 37.5 Hz. The noise floor is typically 3 to 5 times higher at 5 Hz than it is at 37.5 Hz. Noise increases approximately as 1/frequency. Doubling the operating frequency can improve SNR by a factor of 3-5 or more.
Flow tubes are manufactured in a wide range of pipe sizes. Larger flow tubes, such as 24 inch diameter flow tubes, are difficult to drive at higher coil frequencies because they have high inductance. The large coils are slow to respond to a change in coil current direction due to their high inductance and resistance.
A method and magnetic flow transmitter are needed that are compatible with a wide range of flow tube sizes and electrode noise levels encountered in the industrial flow measurement environment.